Various types of traveling web filters are known; U.S. Pat. No. 3,690,466 (Lee) and U.S. Pat. No. 3,731,808 (Rickert) describe travelling web filters in which filtration is aided by generating an underpressure, with respect to atmospheric, in the filtrate chamber. The Rickert structure U.S. Pat. No. 3,731,808) subdivides the filter into a contaminant chamber and a filtrate chamber. In order to ensure that outside ambient air is excluded and does not penetrate into the filtrate chamber and into the contaminant chamber, respectively, which might interfere with the filtration process, complex sealing arrangements are used in those regions in which the filter web passes into the actual filter structure in order to define the respective chambers. These mechanical seals are effected by pressure arrangements, typically hydraulically operated pressure elements sealingly pressed against the filter web by hydraulic cylinders. The vacuum filter in accordance with the Lee U.S. Pat. No. (3,690,466) does not utilize a contaminant chamber; rather, the liquid to be filtered is maintained in form of a pool above the filter band or web. Such an arrangement cannot be used to employ the pressure above the filter web in order to control a filtration process.